Ink-jet printers have become widely accepted as reliable and inexpensive means of high-quality printing. A typical ink-jet pen has a print head having a plurality of nozzles through which ink droplets are ejected. Adjacent to the nozzles are ink firing chambers where ink is stored prior to ejection. Ink is delivered to the firing chambers through ink channels in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir part of the pen. During printing, ink located in the firing chamber is heated or vaporized by a heat transducer, such as a thin film resistor. Formation of the ink vapor bubble is known as nucleation. The rapid expansion of the vaporized ink forces a drop of ink through the nozzle.
One type of ink-jet printer includes a carriage that is reciprocated across a sheet of paper that is advanced through the printer. The reciprocating carriage holds a pen very close to the paper. The pen is controlled by the printer for selectively ejecting the ink drops from the pen while the pen is reciprocated or scanned across the paper, thereby to produce characters or an image on the paper. Typically, when carried on a reciprocated carriage, the pen will have a small reservoir for holding a limited amount of ink. A relatively larger supply of ink is provided in a stationary container that is mounted to the printer and is permanently or occasionally connected to the pen.
An important design consideration for ink-jet printers is to maximize the printing speed. One method of increasing the speed of the printing operation is to increase the velocity with which the pen is scanned across the paper. Reducing the weight of the pen, including the ink reservoir connected to the pen, permits high velocity scanning of the pen while minimizing the power requirements of the motor that drives the carriage.
In order to print effectively, the firing chambers and nozzles need to be "primed" with ink. Typically, priming includes moving ink into the firing chambers. Ink is moved to and held within the chambers and nozzles by capillary force. Priming does not occur spontaneously as ink is first added to a pen. Air bubbles lodged in and around the firing chambers may act to prevent spontaneous priming. Priming tends to be even more problematic in pens that store ink under a slight back pressure. As used herein, the term "back pressure" means a partial vacuum within the pen. In such systems, the presence of a back pressure ensures ink is expelled only when the print head is activated (i.e., when ink is ejected). However, the slight back pressure is not so high as to impede the movement of ink into the firing chambers and nozzles.
A specific priming operation is usually provided to prime the print head of an ink-jet pen. Such priming usually takes place in ink-jet pen factories by inverting the pen after it has been filled with ink and sucking air and ink through the print head nozzles. While such factory priming works well, it has a number of disadvantages. For instance, special low-water-loss packaging is required to prevent nozzle dry-out in factory-primed pens. In addition, factory-primed pens have a limited shelf life of about 18 months, after which the ink quality may degrade due to water loss. Additionally, print head adhesives that are used for sealing the nozzles prior to use may fail due to corrosive characteristics of the ink. Moreover, a factory-primed pen, once installed in a printer, is not designed for repriming in the event that one or more print head nozzles become de-primed.
Systems for priming ink-jet pens while the pens are installed in a printer have been developed. These systems solve some of the problems associated with factory-primed pens. Such in-printer systems usually prime pens by sucking ink outwardly through the nozzles of the pens. However, existing in-printer priming systems have disadvantages. For example, the ink that is sucked through the nozzles is sometimes absorbed by a disposable absorbent pad. Thus, ink is wasted and periodic maintenance to replace the absorbent pad is required.
After initial priming of a print head, care must be taken to eliminate air bubbles that are later introduced to or formed within the ink-jet pen. Air bubbles may be introduced when carried in the ink supplied to the pen. Air is diffused throughout most inks. Heat, either ambient or generated by the ink-jet pen, causes dissolved air within the ink to form air bubbles within the pen. Such air bubbles do not readily redissolve when the ink cools. Additionally, air may be introduced to the pen through the nozzles during ink droplet ejection. That is, after the drop is ejected, ink remaining in the firing chamber is drawn back in toward the print head, drawing with it air from outside the print head. Air may also be introduced to the system should the pen be dropped or bumped. Also, the process of capping the pen to prevent ink from flowing through or drying within the nozzles when the pen is not in use may force air into the print head.
The presence of air bubbles within the pen usually leads to print quality problems. An air bubble can obstruct ink flow to particular firing chambers from which ink droplets are to be ejected. Air bubbles can cause irregularly shaped ink droplets or cause a print head to deprime resulting in complete failure of the print head. Consequently, ink-jet print heads should be substantially free of air.
Previous attempts to eliminate the problems caused by air bubbles within the pen system have included storing the air at a location known as the standpipe that is located between the print head and the ink supply or reservoir. However, to allow ink to flow around air stored within the standpipe, the size of the standpipe must be relatively large, in turn requiring an undesirably large pen.
In addition to air bubbles, excessive heat within the ink-jet pen can lead to print quality problems. As discussed above, heat within the pen system may liberate dissolved-gas bubbles in the ink. Additionally, excessive heat within the pen may cause prenucleation of the ink vapor bubble resulting in poor ink droplet formation. Excessive print head heat can also change the composition of the ink through evaporation of various ink components. Such changes in the ink composition may also cause poor droplet formation.
The present invention addresses the above-described traditional thermal ink-jet problems with a single ink delivery system. The present invention provides thermal management within the pen, removal of air bubbles within the system and priming of the ink-jet pen all while the pen is installed in a printer.
According to the present invention, a preferred ink delivery system for an ink-jet pen comprises a pen cartridge having an internal ink reservoir. The cartridge has a fluid inlet and a fluid outlet, both in fluid communication with the reservoir. A circulation conduit connects the fluid outlet and fluid inlet. The circulation conduit permits ink circulation into the fluid inlet, through the reservoir, out of the fluid outlet, through the circulation conduit, and back to the fluid inlet. Such ink circulation delivers ink to the print head, dissipates heat generated within the pen, removes air bubbles throughout the pen system, and primes the print head.
Additionally, the present invention provides for initially filling or refilling an ink-jet pen reservoir. The reservoir refilling operation may be continuous wherein ink is continuously circulated through the ink reservoir. Alternatively, the reservoir refilling operation may be periodic wherein ink is only circulated to the pen reservoir when the ink supply within the pen is low.
Moreover, because the ink delivery system of the present invention facilitates the removal of air bubbles from the standpipe as well as from other areas throughout the pen system, the standpipe size may be reduced. A smaller standpipe yields a smaller ink-jet pen. Thus, a smaller ink-jet pen is provided by the present invention without reducing printer reliability, print speed or print quality.
The present invention also allows priming of the print head without requiring either ink or air to be removed through the print head nozzles. Accordingly, ink is not wasted and traditional disposal of waste ink and waste ink absorbent pads is not necessary.